1. Field of the Invention
This invention relates to a method and apparatus for performing two phase high voltage insulation testing of multiphase windings such as those found on large electrical machinery.
2. Description of the Prior Art
At present, American National Standard Institute's Specification C50-10 as issued in 1975 and Institute of Electrical and Electronic Engineers' Specification 95 issued in 1975 are the industry standards for final insulation tests on wound high voltage electrical equipment and provide for applying twice the rated voltage plus 1 kilovolt to each phase for one minute while the other phases are grounded. As the demand increases for equipment with larger power ratings over the previously available units, the need for increasing the operating voltages of these machines becomes more apparent since the present level of load currents are near the maximum allowable limits. This is due primarily to the magnitude of the electromechanical forces involved with such high currents.
Limitation of the present high voltage test procedures becomes more significant as the rated voltages of large electrical equipment increase beyond 20 KV. In fact, at the present time, the size of electrical equipment such as a generator, for example, is substantially increased in size in order to obtain the necessary clearance and voltage holding capabilities for testing. This increase in size is not required for normal operating circumstances, especially for a generator which is operated in a hydrogen environment of four or more atmospheres.
The basic guideline for industry's standards for direct current insulation testing of large electrical equipment such as large AC rotating machinery is provided by the Institute of Electrical and Electronic Engineers, Specification No. 95, as issued 1975. The purpose of the guide is to provide uniform procedures for performing high direct voltage acceptance tests and routine maintenance tests on the main ground insulation of windings on large electrical machinery. In addition, the procedure provides uniform procedures for analysis of the variations in measuring current so that any possible relationship of the components of these variations to the conditions of the insulation can be more effectively studied.
The procedure provides for the application of a test voltage to both ends of one winding. The remaining windings are then grounded. The test voltage is applied for a period of one minute during which time the current flowing from the test voltage power source to the winding is monitored. It is through the monitoring of the current that a failure can be detected. A complete failure is usually indicated by a sharp capacitive discharge at the point of failure. There are times, however, when failure or partial failure may be indicated by a large abnormal change in the current reading or by erratic current indications being observed.
In addition to being able to perform the voltage test as described above, the procedure provides for performing a controlled overvoltage test, sometimes referred to as a "direct current leakage test" or a "step voltage test". This test is a high direct voltage test in which the voltage is increased in a specified manner during which time the measured current is observed. This type of test, done under suitable conditions, provides a record of the condition of the winding for present and future use and may predict breakdown voltage if it is within or slightly above the test voltage. Conclusions are reached by recognition of abnormalities or deviations in the curve of microamperes versus applied volts, plotted as the test progresses.
The controlled overvoltage test, initially involves the application of approximately one-third of the maximum test voltage. The next step is the determination of polarization index which is the ratio of the one minute current reading to the ten minute current reading of a particular winding. After the application of the voltage, the current is noted at the expiration of one minute and at ten minutes. The quotient that is derived by dividing the ten minute reading into the one minute reading is called the polarization index.
After the initial ten minute step, the uniform voltage test steps are conducted. The voltage is raised on the winding under test approximately one kilovolt per step. The voltage is held at this level for approximately one minute and then the test proceeds until the equivalent of the maximum test voltage of two times the rated voltage plus 1 KV is achieved. The test is then conducted on the next winding and repeated until all of the windings have been tested.
As an alternate to the above test method, IEEE Standard No. 95 provides for the situation where it is desirable to obtain only the true leakage current on a control overvoltage test. However, procedures require compromise to prevent allowing too brief a time at each voltage step or maintaining each voltage for a long time so that the absorption is practically complete and only the leakage current remains. The gradient time method uses a reasonable time and provides a result related to the true leakage current components.
This test procedure requires the initial application of a voltage step of approximately 30% of the maximum test voltage. This voltage is maintained constant for a period of ten minutes, during which time the measured current is observed and logged at 1 minute, 3.16 minutes and 10 minutes. The 3.16 minute reading may be extrapolated from a curve of the plotted current readings over the ten minute period. It is then required that the conduction component C of the measuring current be calculated by the equation: ##EQU1## where C is equal to the current at 1 minute times the current at 10 minutes minus the current at 3.16 minutes squared, the total being divided by the sum of the current at 1 minute plus the current at 10 minutes, minus two times the current at 3.16 minutes. By subtracting the value of C from the current at 1 minute and the current at 10 minutes the absorption currents for these current readings can be calculated. The absorption ratio is then determined by dividing into the absorption current at one minute, the absorption current at 10 minutes. By calculating the absorption ratio as described above, there are tables available such as Table I, which is taken from IEEE No. 95, which relate the absorption ratio to the amount of time that the remainder of the test voltages must be applied at each step. After the absorption ratio is determined, the test voltage is increased by a value of 20% and held for the period of time that is from Table I and each step thereafter is performed in a similar fashion.
There has been much work performed in the area of testing multiphase windings. Of these, U.S. Pat. No. 2,890,410 provides a method of testing a three phase machine by applying DC impulses of equal magnitude and opposite polarity simultaneously to each of two windings of the machine. A voltage detecting device is applied between either the neutral point or to a third terminal and ground. If no fault exists in the windings, the voltage pulses will cancel each other out at the voltage detecting point and the detector will read zero voltage. If a fault does exist, the voltages will be unbalanced and the detector will indicate a fault.
In U.S. Pat. Nos. 2,321,424 and 2,815,481, there is disclosed a method of testing by which DC surge voltages are applied across two windings of a three phase machine in order to test its insulation characteristics. The voltages are placed in series relationship between the test windings and ground, and the test voltage is above and below ground potential. The third winding is not directly grounded.
In U.S. Pat. No. 2,558,091 there is disclosed a method for testing for a discharge on high voltage windings where a voltage is applied across two windings. Here the applied voltage is AC with the winding being connected between the voltage source and ground. Also, there is additional test apparatus serially connected between the second winding and ground.
It is important that adequate testing be performed on new machinery to determine the sufficiency of each insulation mode, i.e., line to line and line to ground. It is felt that a more preferred test can be accomplished by applying to two phases simultaneously a positive and negative voltage with the remaining phases grounded. With a series of such tests, a complete machine insulation system could be proved adequate for the application for which it was designed by applying a lower test voltage than the current industry standard of two times the rated voltage plus one kilovolt.